Saturday, July 5, 2014

A Midsummer Night's Blog Post

WHERE DID ALL THE TIME GO!?

No matter. 

IGBTs! IGBTs! IGBTs!

LOOK AT THESE IGBTs!



Current limited by the package leads, among other things, these are the perfect candidate for some pulsed, hard-switched, power dense goodness.

And so the journey to building a QCW bus modulator begins.

In essence, the modulator portion of your QCW coil is just a buck converter capable of producing arbitrary waveforms across the bus of your DRSSTC.

The tried and true control scheme for these guys is hysteresis (bang-bang) control, which involves turning on the high side of your buck converter when the sense voltage falls below your hysteresis band and turning the high side on when your sense falls below the hysteresis band.

In lieu of a microcontroller, I opted to make my control loop out of op-amps for noise immunity and beardy-weirdy cred.


Troller Schematic. Forgive me, for I have text overlap up the wazoo. 
The first two op amps are set up as an adder and differentiator, which produce the hysteresis band given the input waveform. These signals are then passed to the latter two, which acts as a window comparator.

The outputs of the comparators are then fed to an SR latch so that the hysteresis thing happens.


Squiggles of Science
The control loop can then be tuned using the width of your hysteresis band or some fancier processing downstream.

Eagle'd:



Special notes: the input signal is actually taken from an opto, which is filtered by an LC to reproduce the desired waveform. It's like L-C filterception. Additionally, this allows me to send square wave pulses optically, instead of an analog signal through a long length of coax, which is bound to cause massive damage pick up noise.

Here it is in board form:


Note: not fully routed not the actual board
After doing some math and running the simulation for the bus at full load in spice, I concluded that I simply couldn't find bus caps large enough and in the desired package, so, the some revisions were made to the design.

The boards are now split up into three sections: a DC-chopper motherboard, controller daughterboard, and DRSSTC with filter LC.


Can't have too much bus cap (a work in progress)
By moving the filter LC to the DRSSTC bridge, I could then populate the DC-Chopper with as much bus cap as my heart desired (and keep the ripple voltage below 5%).

The addition of the driver daughterboard would allow me to send out for plenty of driver revisions without having to spend a fortune on the whole 5.5" x 5.5" board. Oh, and did I mention how small everything is? :3

Current limiting is implemented on the driver side as desaturation detection: in an over-current state, the voltage drop across the switches increases to well above the typical ~2.0V, which is detected by the drivers, which do some fancy soft-turn off to prevent voltage spikes induced by the typically high dI/dt. The gate drivers also conveniently have built-in opto-isolated inputs.

IN OTHER NEWS:

Bluescooter got an overhaul to make it more of a reliable commuter vehicle than junkyard scrapper on the verge of collapse.

After eating through a current modded controller, a few things were changed:

1) Motor upgrade!


Sweet Bajeesus
After trolling around the internet for a sufficiently large motor, I had at some point chosen this guy:

http://www.hobbyking.com/hobbyking/store/__18179__Turnigy_Aerodrive_SK3_6354_215kv_Brushless_Outrunner_Motor.html

An especially squat, low kv motor, perfect for squeezing into the tiny 4" u-channel frame. 


But everything changed when the fire nation attacked the motor went on backorder.

I was forced to look beyond Ye' Old Hobbyking, and found a cheaper, dare I say, better option:

http://www.sdshobby.com/emp-n-series-outrunner-brushless-motor-n6354200kv-p-2486.html

Even with expedited shipping, the SDSHobby motor lends itself to be a good $20 cheaper than the Hobbyking equivalent, the only downside from ordering from SDSHobby being their rather small selection of EV-sized motors.

2) The controller with the infinite heatsink


After eating through a controller, and being too lazy to replace the dead FETs, I opted to avoid yet another headache by heatsinking the controller to the chassis.




Some fresh thermal grease, three holes, and some cap screws later, a happier 'troller was born.
Even on especially hot days, I've yet to find the area around the FETs go more than 5°C above ambient. 


3) It has a new caddy. 



Structural Hot Glue


Made entirely out of mystery plastic polypropylene and a bit of polycarb from MITERS. The key switch was also moved to the outer face of the port panel to accommodate for the larger motor. 


Fin.

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